Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.

Keeping state or being stateful means that some device is keeping track of another device or a connection, either temporarily or over a long period of time. When I put someone's name in my address book and note their birthday and phone number, one could say that I am maintaining state for that person. On the Web, a cookie is a stateful mechanism that allows Web servers to keep track of information about people, as described in a moment.

A stateful connection is one in which some information about a connection between two systems is retained for future use. In some cases, the connection is kept open even though the two systems might not be transmitting information (i.e., the connection itself retains state).

In contrast, a stateless connection is one in which no information is retained by either sender or receiver. The sender transmits a packet to the receiver and does not expect an acknowledgment of receipt. The recipient receives the packet without any prior connection setup.

A TCP connection-oriented session is a stateful connection because both systems maintain information about the session itself during its life...